The invention relates to a method and device for a tandem drive, or multiple drive axles for a vehicle. The invention is in particular directed to detection of differences between the rolling radii of wheel pairs on different driven axles in a tandem or triple axle arrangement, i.e. in a double or triple bogie arrangement. The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may be used for any kind of vehicle in which there is a double or triple bogie arrangement including two driven axles present.
For certain vehicles there is a desire to provide the vehicle with more than one axle at certain locations. This feature is in general due to a desire to distribute the weight of the vehicle to several tires and the vehicles may be provided with double bogies or triple bogies, i.e. two or three axles whereof each one provided with wheel pairs located adjacent to each other.
There is in many cases a desire to be able to provide a propulsion force to at least two of these axles such that a so called tandem axle is used. In a tandem axle are two axles connected to a drive shaft and both axles may provide a propulsive force for the vehicle. In case of having 3 axles, and three wheel pairs in the bogie arrangement, could all three axles be driven axles in a so called triple axle. The set up in triple bogie arrangement could also be that two of the axles and its associated wheel pairs form a tandem axle while the third axle is a non-driven axle.
When using several driven axles connected to each other in a tandem axle or triple axle arrangement may the arrangement provide an efficient traction which in particular is useful when the vehicle is heavy loaded and/or at take-off and/or in uphill condition and/or travelling on loose or slippery surface conditions. The use of several pairs of traction wheels during these conditions may be of great advantage. However, during travel conditions on high-ways or under ordinary road conditions in general may there be a problem in using a tandem axle or triple axle since they are driven by the same drive shaft and thus propelled to rotate with the same rotational speed. In this case, even small differences in the rolling radius of the wheel pairs of a tandem axle or triple axle may cause a rather large wear of the tires and induce stress in the driven axles. One way of reducing this problem is to use a longitudinal differential (also commonly called an inter-axle differential) working between the driven axles such that they may rotate at different speed by the use of the longitudinal differential. However, to include an extra differential, to allow a speed difference between different axles, in the wheel suspension arrangement will contribute with an additional cost as well as a more bulky and heavy arrangement which could be hard to fit in the suspension arrangement. In US 2002/0179345 is described a tandem drive axle arrangement which is designed to work without a longitudinal differential working between front and rear driven axles in the tandem axle. It is therein further described how differences in the tire rolling radii is monitored in order to control tire pressure for correcting axle speed differences.
Even though US 2002/0179345 discloses a tandem axle system and a method for operating such a system in order to reduce undesired wear of tires while also providing an efficient propulsive force to the vehicle is it desired with an improved system being able to provide a desired traction force for a multitude of driving conditions and vehicle parameters.
It is desirable to provide a wheel suspension system comprising a driven multi-axle and a method for controlling such a system, which system and method provide for an improved functionality and efficiency under a wide variety of driving conditions.
A wheel suspension system according to an aspect of the present invention comprises a first axle, provided with a first wheel and a second wheel, and a second axle, provided with a third wheel and a fourth wheel. This first and second axle are driven axles and forms part of a tandem axle or a multiple axle. By tandem axle (or multiple axles) is meant a propulsion arrangement in which two axles (or multiple axles) are powered by a common drive shaft system. In the following, the system will be focused on tandem axle system even though it is understood that there may be further driven axles included in the wheel suspension system. Hence, the driveshaft comprises a first portion connected to the first axle via a first differential and a second portion connected to the second axle via a second differential. These differentials are thus arranged to work transversally, i.e. to allow differentiated speed of the wheel pairs of the respective wheel axles and not to differentiate the speed between the different axles.
The wheel suspension system further comprises angular speed sensors designed to detect the rotational speed of the first and second axles and/or the rotational speed of the respective wheels of the first and second axles. By angular speed sensors is meant any kind of sensors being able to sense or indicate a parameter which can be used to compute or detect the rotational speed of an axle and/or wheel. The angular speed sensors used could for example be the same sensors which are used for an Anti-locking Brake System (ABS) for detecting locking of wheels when braking or for detecting slip in traction control. It could also be other kind of angular speed sensors mounted on the wheel axles, wheels, gearbox, engine or other parts of the powertrain suitable for detecting the angular speed of the axles and/or wheels.
The angular speed sensors are connected to an electronic control unit (ECU) which receives input from the angular speed sensors. The ECU is designed to use the input from the angular speed sensors in order to calculate a difference between the angular speed of the first and second axles and/or a difference between the angular speed of the respective wheels of the first and second axles. Depending on the information from the angular speed sensors, and possibly further information concerning vehicle and/or driving conditions, the ECU can be programmed to send output signals to control the wheel suspension arrangement or to alert or inform a driver of appropriate measures to be made in response to the prevailing conditions.
The wheel suspension system also includes a coupling arranged in the driveshaft and positioned between said first and second drive shaft portions for changing the first and second drive shaft portions between being drivingly connected and disconnected. The coupling can comprise any suitable device, such as a claw clutch, a dog clutch or a friction/disc clutch, which may change between being disengaged and being positively engaged. Hence, this feature enables the possibility to completely disconnect the first and second driven axle from being drivingly connected. This feature involves a number of advantages compared to the system disclosed in US 2002/0179345. An obvious advantage is that it is easy to measure the angular speed for the first respective second axle with a better accuracy. In US 2002/0179345 the driven axles are constantly connected whereby it should be a more delicate problem to accurately measure a difference in the rotational speed of the axles and/or wheels by angular speed sensors. Since the axles are connected together and there is no differential working between the axles, the axle speed should be the same for both axles and the wheels are forced to slip or spin. In addition, the possibility to disconnect the axles means that the coupling can be disconnected if it is indicated that the angular speed difference between the axles, or wheel, are above a prescribed value. There will in this case be a loss of traction force from the disconnected axle but the vehicle may avoid to be subjected to friction forces caused by a pair of driven axles being in a condition where they should rotate at different speeds in order to represent the same vehicle speed.
In order to further improve the functionality of the wheel suspension arrangement it could be provided with a lifting mechanism acting on one of the driven axles, e.g. the first axle. The lifting mechanism is designed to be able to lift the axle and shift the position of the axle from a first position in which its associated wheels are in contact with the ground and a second position in which the associated wheel pair is lifted above the ground surface so as to not be in contact with the ground. It should be noted that in the subsequent text the term “first axle” can serve as an example of a liftable axle. However, the liftable axle can be located as the foremost axle, or pusher axle, (closest to the front portion of the vehicle), as the rearmost axle, or trailing axle (closest to the rear portion of the vehicle), or as any axle between the foremost and rearmost axles. The latter example relates to cases where the wheel suspension arrangement comprises more than two axles. To be noted, more than one axle can be liftable and the invention disclosed herein is suitable for any combination of liftable and non-liftable axles in the wheel suspension arrangement.
This feature will thus enable the possibility to use the lifting action of an axle out of contact with the ground, if it is decided that the use of the first and second axle for propulsive purposes is considered not suitable, e.g. due to a difference in the rotational speed of the axles for the same vehicle speed. However, in many cases is it more desired to control the liftable axle, e.g. the first axle, to be maintained in its ground engaging, lowered position due to a desire to use an additional axle for load sharing. In this case the liftable axle can be used for dividing the load between axles and be controlled to not be drivingly connected to the engine while another axle, e.g. the second axle, is drivingly connected to the engine via the driveshaft. Lifting and lowering of the liftable axle, as well as the connecting/disconnecting of an axle or axles is normally performed when the vehicle is stationary. Typically, the axle is lowered and connected prior to loading, in order to avoid overloading other axles, and is lifted and disconnected after unloading, in order to reduce tire wear and to facilitate launch under slippery conditions. Allowing measurement to be performed by, for instacne, delaying axle lift or axle connection is possible but undesirable for the said reasons. However, if the axle or axles to be measured are connected at the time of launch and/or the vehicle is travelling with a lowered axle due to the current load, then measurement can still be carried out by performing a brief, temporary disconnection of one or more shafts to allow measurement of angular speeds. Such a measurement is preferably carried out when the transmission is operated at a light load, e.g., when travelling at constant speed on a flat road or a slight negative grade.
Lifting an axle out of contact with the ground is however beneficial when there is no problem with different axle rotational speed with respect to the driven axles and can be used to save fuel in ordinary driving modes. However, the tires of the lifted wheels will be subject to less wear and it may therefore be even more important to include the feature of keeping track of different angular speed of the axles in this case, since an angular speed difference may arise from the tires of the respective axles being used differently or being worn at different rates.
As mentioned earlier, the wheel suspension system may comprise a third driven axle connected to a third pair of wheels (a fifth wheel and a sixth wheel) which can also be provided with angular speed sensors designed to detect the rotational speed of the third axle and/or the rotational speed of the respective wheels of the third axle. This third axle is connected to the same driveshaft as the first and second axle. The third axle is connected to the drive shaft system via a third differential for allowing the third wheel pair to rotate at different speeds. The system can be provided with a second coupling in the driveshaft for being able to disconnect the third axle from the first and/or second axle. In this example, the first axle, which serves as an example of a liftable axle, could be the foremost, or pusher axle, the rearmost, or trailing axle, or the intermediate axle located between the pusher axle and trailing axle. In the case of the pusher axle being a liftable axle, a further coupling can be provided between the pusher axle and the drive shaft (transmission output shaft). The vehicle can then be driven by a driven front axle or by a driven axle located to the rear of the pusher axle and having a separate connection to the drive shaft. If desired, the wheel suspension system may comprise further driven axles as well as further non-driven axles.
As indicated above, the wheel suspension system may comprise at least one driven front axle connected to a fourth pair of wheels (a seventh wheel and an eighth wheel) which can also be provided with angular speed sensors designed to detect the rotational speed of the driven front axle and/or the rotational speed al the respective wheels of the driven front axle. This axle is connected to the same driveshaft as the two or more driven rear axles. The driven front axle is connected to the drive shaft system via a fourth differential for allowing the fourth wheel pair to rotate at different speeds. The system can be provided with a third coupling in the driveshaft for being able to disconnect the driven front axle from the driven rear axles. The system can be designed such that each driven wheel is provided with a respective angular speed sensor for detection of the angular speed of each wheel. By including this feature it will be possible to evaluate the actual angular speed of each wheel and detect not only inconsistencies between the speeds of different axles but also keep track of each individual wheel when performing the control of the wheel suspension system. The ECU may thus indicate or send out control signals detection of conditions relating to specific wheels.
The wheel suspension system can be used for and comprised in a multitude of vehicles. It is in particular suitable for trucks designed to be heavy loaded and/or to be used for vehicles performing heavy work in poor road conditions.
The invention also relates to a control method for a wheel suspension system including a multi-drive axle, e.g. tandem axle, for a vehicle. The wheel suspension system comprises at least two driven axles whereof each axle is provided with a pair of wheels. The axles are connected to a driveshaft via differentials and the driveshaft comprises a coupling positioned between each differential connected to the driveshaft so as to divide the driveshaft into different driveshaft portions. The controllable coupling (or couplings, if several are present) can change between being engaged or disengaged such that the different driveshaft portions and its connected driven axles switch between being drivingly connected and disconnected to each other. The wheel suspension system further comprises angular speed sensors designed to detect the angular speed of the axles and/or said wheels connected thereto. The control method comprises the following steps, which is exemplified for a tandem axle, i.e. a wheel suspension system comprising two driven axles.
In a first step the coupling is controlled to be disconnected such that a first drive shaft portion, comprising a first differential connected to a first driven axle, is drivingly disconnected from a second drive shaft portion, comprising a second differential connected to a second driven axle. In this state may thus the first and second axles rotate without influencing the speed of the other axle. In general will this mean that one axle is totally disconnected and may rotate freely independent of a power source, e.g. an Internal combustion engine (ICE), while the other axle is connected to a power source. Also the other axle could be disconnected to reduce the risk for slip and thus being able to measure a correct value of the angular speed of that axle. However, the other axle could be still connected to the power source and the measurement could be set to take place during a sequence when a driving or braking torque are within prescribed values in order to avoid slip or spin of the wheels when performing the measurements. As can be readily understood, a suitable occasion for performing measurements is when there is no torque applied at all, neither propulsive nor braking torque.
In a subsequent step, the angular speed of the axles and/or the wheels connected thereto is detected by the angular speed sensors while the driven axles are drivingly disconnected from each other.
In a subsequent step, the detected angular speed of the axles and/or the wheels connected thereto is used in order to calculate an angular speed difference between the angular speed of the first and second axles and/or a difference between the angular speed of the respective wheels of the first and second axles.
In a further subsequent step, the calculated angular speed difference between the different axles and/or wheels is compared with reference values to establish a comparison of the speed differences.
In a final step, the comparison of the angular speed difference is used as an indication of different wheel radius and triggers a warning signal and/or a control action in case the angular speed difference is outside an allowable value. By indication of wheel radius is herein meant to include measurements or indications of the circumference of the wheels, rolling radius or other parameters being related to the wheel radius. There can be different control actions depending on the magnitude of the difference and/or driving conditions and vehicle characteristics. It may for example be allowed, or indicated to be suitable, to use both the first and second axle for propulsive power if the vehicle is driving at rather low speed on loose ground even though the speed difference of the axles are above a certain limit while it is indicated that both axles not should be used when driving on an ordinary road above a certain speed limit, even though it can be allowed to drivingly connect both axles. Above higher levels of speed difference, and possibly also in dependence of road or driving conditions, could a control system prevent a pair of axles having too large speed differences to be allowed to be drivingly connected.
A speed difference between a pair of axles may also be used as an indication of tire wear and could for example alert a driver to check the tire status. Hence, the result of the measurements of the angular speed could be either a direct control signal to the wheel suspension system (or to some part of a vehicle) or to alert a driver of a status or feature to be checked.
The control method described above can be used for a wheel suspension system comprising a lifting mechanism acting on one of the axles, e.g. the first axle. The lifting mechanism could thus be designed to be able to lift the first axle in order to shift the position of the first and second wheels between being in a working position in contact with the ground surface and being lifted up to be in a resting position above a ground contact level.
In case the wheel suspension system comprises a lifting mechanism as described above, the control method may include the following steps to precede method steps described above. In a first step the first axle is lowered from an up-lifted resting position, in which the wheel pair of the first axle is above a ground contact level, to a lowered working position, in which the wheels are in contact with the ground. This operation is performed while the coupling is disconnected such that the first axle is drivingly disconnected from the second axle. In the next step, the coupling is maintained disconnected to keep the driven axles drivingly disconnected. The axles are drivingly disconnected at least until it is indicated that the detected angular speed values from the angular speed sensors, indicating the angular speed of the first axle and/or its wheel pairs, are stabilized. The wheel speed is in generally stabilized within one or a few seconds from the moment the wheels come into contact with the ground.
In case the angular speed difference of a pair of axles is outside an allowable value or a critical upper limit, one control action could be to keep these axles drivingly disconnected from each other. This control action could be dependent on further parameters, e.g. load, vehicle speed and road/surface conditions.
In case the wheel suspension system is provided with a lifting arrangement as described above, and the angular speed difference is outside a critical upper limit, the liftable axle (first axle) could be controlled to be lifted to an up-lifted position in which its wheel pair is above ground level and not is in contact with the ground. This control action could be dependent on further parameters, e.g. load and permitted axle pressure. However, even though a wheel suspension arrangement comprises a liftable axle is probably the control action in most cases also for this kind of arrangement, when there is detected a large axle speed difference, to keep the axles drivingly disconnected from each other while keeping the liftable axle in its lowered, ground engaging position such that there may be a load distribution between the axles.
Depending on the magnitude of the calculated speed difference could different control actions be performed or alerts indicated. For example, in case a calculated angular speed difference between the angular speed of the first and second axles exceeds a “change warning limit” is it indicated that a change of position of the tires from one axle to another should be performed. This can be particularly useful for a wheel suspension system in which there is a liftable axle present, such that wear of a pair of wheels on the liftable axle is substantially less than for a pair of wheels which always engages with the ground. However, it may also be possible that one pair of wheels of a certain axle in general is worn more than a wheel pair of another axle also for a vehicle in which it is not possible to lift any axle.
There may of course be further angular speed difference limits indicating different actions or alerts. In case a calculated angular speed difference between the angular speed of the first and second axles exceeds a second limit, a “check warning limit”, is it indicated that a manual check of the conditions of the tires should be performed. This limit could be set to be higher than the “change warning limit” described above. Hence, there can be several different levels or values of angular speed differences between axles and/or individual wheels which may cause different control actions and/or alerts. As previously described, one action when the relative speed difference is rather large is that it is prevented (prohibited) that the axles are drivingly connected. This limit could for example be the same as the second limit (“check warning limit”) mentioned above. At another level, for instance with less speed difference, the limit could be set so that the automatic engagement of the axles is aborted, while a driver is alerted about the speed difference of the axles and the axles can be allowed to be manually controlled to be connected. This limit could for example be the same as the first limit (“change warning limit”) mentioned above.
In case a difference in the angular speed is detected between the axles/wheels, the pressure level of the respective tires can be detected and controlled. For example, if the pressure levels are detected to be quite different, or deviating from a set desired value for the respective tires, could the pressure level be set to be equalized or set according to the desired values. When the air pressure in the tires have been adjusted accordingly could a new measurement of the angular speed of the axles/wheels be made.
The air pressure in the tires could also be used to compensate for differences in the rolling radius of the wheels indicated by differences in the angular speed between different wheels/axles. Hence, the tire pressure for a pair of wheels on an axle indicated to have a slower angular speed could be controlled to be decreased while a the tire pressure for a pair of wheels on another axle indicated to have a faster angular speed could be controlled to be increase in order to compensate for speed differences between the axles. The relative axle pressure of the axles could also be controlled in dependence of detected angular speeds of the axles to compensate for speed differences.
In addition to compare absolute values of the angle speed difference could it also be taken into account if there is a sudden change of the angular speed difference between different axles or different tires, e.g. by keeping a track record of the differences detected. In this case an alert or warning can be outputted from the ECU if there is a drastic change in the angle speed difference which does not seem to be reasonable when compared to previous measurements. A rather drastic change for a single wheel in its angle speed could for example be an indication of a lowered tire pressure due to a leakage of air from the tire or a sudden pressure loss due to a puncture.
The angular speed can be measured on the axles or on the individual wheels. There is in general an angle speed sensor present in the gear box, the signal from which is proportional to the angular speed of a driven axle, at least if the braking or propulsive torque is rather low and the vehicle is driving under ordinary road conditions. In this case could it be enough to add an angle speed sensor to those axles which must be disconnected from the driveshaft when performing the angle speed measurements. However, in many cases angular speed signals are already present for each wheel from the anti-locking brake system (ABS) for the brakes. Hence, an angular speed can be detected by wheel speed angular sensors for wheels on each side of the axle, for all axles being connectable to the driveshaft in the wheel suspension system. The angular speed for each one of said wheels are used in order to calculate an angular speed difference between said axles. In this case the available sensors can be used.
In order to perform the angle speed measurements certain criteria could be set to get reliable values. The system can be controlled to detect the angular speed of said axles and/or the wheels connected thereto under certain conditions, or to at least indicate if there are conditions present when performing the measurements which may influence the values to not be suitable to be used for calculating an angular speed difference between the angular speed of the first and second axles and/or a difference between the angular speed of the respective wheels of the first and second axles. Hence, when performing the measurements it is generally considered that if one or several of the following criteria are fulfilled will the resulting measurement be more reliable:                the turning radius of the vehicle is above a prescribed limit, i.e. the vehicle should preferably be driving essentially straight. It is of course possible to compensate for the difference of travelling distance for the different wheels during turning but it is in general better to perform the measurements when driving essentially straight ahead;        the speed variations of the vehicle are below a prescribed limit. To this criterion could be added that inclination and/or the curvature of the road should be within certain limits. In order to achieve this the measuring time point could be selected by using prediction of the upcoming driving sequence, e.g. by using route or traffic information. For example, GPS and map data in order to select a suitable location or stretch of road for performing the measurements, for instance an essentially flat and straight road portion;        the tire pressure difference between different wheels is within a prescribed limit. There are indications that the tire pressure not is essential for the angle speed, at least as long as the tire pressure difference are within certain limits, but it is advisable to check the tire pressure for the tires in case of unexpected detected values for the angle speed;        the load distribution between the different axles is within a prescribed limit. Likewise, as for the tire pressure, the load on the axle can influence the angle speed and should be checked if the measurements seem to result in unexpected values;        the wheels are not being controlled by an anti-locking braking system or a wheel slip control system while the angular speed is detected. This feature can influence the measurements of the angle speed and the measurements should preferably not be performed while either of these systems are in operation;        the braking force on the measured wheels is below a prescribed value. If the brakes are applied, a slip condition may exist and the values of the angle speed measurements can be inadequate;        the driving torque on the measured wheels is below a prescribed value. If there is strong driving torque, there is risk for spin on the driven wheels which can result in inappropriate values of the angular speed for a wheel or an axle;        the tire temperature, or tire temperature difference, is within a prescribed limit. In general, the temperature of the tires should be about the same but there may be a difference in temperature if one set of tires not have been used for a while (e.g. being lifted up) and the other wheel pair, or pairs of wheels, have been ground engaged and heated due to frictional heat from the road surface while driving; or        the axle differentials should be free, i.e. the differentials working on an axle to allow speed compensating between a left and right wheel on the same axle should not be locked. In locked condition, no speed difference is allowed for the respective wheels (left and right) of an axle        
The invention also relates to a computer program comprising program code means for performing the control steps described herein when said program is run on a computer.
The invention is also encompassing a computer readable medium carrying a computer program comprising program code means for performing the control steps described herein when said program product is run on a computer.
The invention further includes an electronic control unit (ECU) for controlling a wheel suspension system and the electronic control unit (ECU) is configured to perform the control steps as described herein.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.